Heretofore there are various known constructions for a speed sensing rolling bearing unit for supporting a wheel of a vehicle on to a suspension unit so as to be freely rotatable, and for sensing the rotational speed of the wheel in order to control an anti-lock braking system (ABS) or a traction control system (TCS).
With all of the rotational speed sensing units incorporated into these speed sensing rolling bearing units, there is provided a tone wheel which rotates together with the wheel, and a sensor which outputs an output signal which changes at a frequency proportional to the rotational speed of the tone wheel.
In U.S. Pat. No. 5,200,697, special consideration to the side of sensor is not taken.
The sensor disclosed in U.S. Pat. No. 5,293,124 has a shortage that the magnetic flux emanating from the north poles is inclined to leak from the core metal to the south pole resulting in inefficient magnetic flux across the coil and inefficient output voltage.
The sensor disclosed in U.S. Pat. No. 5,004,358 has a shortage that the dual structure of radially outer and inner sections causes the structure complicated and enlarged
In Japanese Hatsumei Kyokai Technical Report No. 94-16051, there is disclosed a speed sensing rolling bearing unit such as shown in FIG. 1.
With this unit, a flange 2 for attaching a wheel, is formed on an outer peripheral surface of an axially outer end portion (here the term "outer" means the widthwise outer side of the vehicle when fitted to a vehicle; the left side in FIG. 1) of a hub 1 constituting an inside member, and an inner ring raceway 3a and a step 4 are formed on an outer peripheral surface of a central portion. Furthermore, an inner ring 5 formed with an inner ring raceway 3b on an outer peripheral surface thereof and constituting an inside member together with the hub 1, is externally secured to the outer peripheral surface of the hub 1 with an axially outer end face thereof abutted against the step 4. Instead of having the inner ring race way 3a formed directly on the outer peripheral surface of the hub 1, an inner ring (not shown in the figure) separate to the hub 1 may be formed, and this inner ring and the inner ring 5 may be externally secured to the hub 1.
A male threaded portion 6 is formed on a portion near the axially inner end of the hub 1. The inside member is made up by securing the inner ring 5 to a predetermined portion on the outer peripheral surface of the hub 1 by means of a nut 7 which is threaded onto the threaded portion 6 and tightened. Furthermore, an outside member 8 which is located around the hub 1, is provided on a central outer peripheral surface thereof with an attachment portion 9 for securing the outside member 8 to a suspension unit. Moreover, outer ring raceways 10a, 10b are formed on the inner peripheral surface of the outside member 8, facing the respective inner ring raceways 3a, 3b.
Furthermore, a plurality of rolling members 11 are respectively provided between the inner ring raceways 3a, 3b and the outer ring raceways 10a, 10b, so that the inside member can rotate freely inside the outside member 8. With the example of FIG. 1, balls are used for the rolling members 11. However in the case of a rolling bearing unit for heavy vehicles, taper rollers may be used for the rolling members.
A seal ring 12 is fitted between the inner peripheral surface at the axially outer end of the outside member 8 and the outer peripheral surface of the hub 1 to cover the opening at the axially outer end of the space existing between the inner peripheral surface of the outside member 9 and the outer peripheral surface of the hub 1, in which is provided the plurality of rolling members 11.
A base end portion of an encoder 13 (left end portion in FIG. 1) is externally secured to a portion on an axially inner end portion (inner here means the side towards the center of a vehicle when fitted to a vehicle) of the inner ring 5, spaced away from the inner ring raceway 3b. The encoder 13 is formed in an overall annular shape (short cylindrical shape) from a ferromagnetic metal plate such as steel plate. The encoder 13 is made up with a smaller diameter portion 14 and a larger diameter portion 15 formed concentric with each other and connected by a stop portion 16 With the encoder 13, the larger diameter portion 15 is externally fitted to the outer peripheral surface of the end portion of the inner ring 5, and is secured to the inner ring 5 with the stop portion 16 abutted against the end rim portion of the inner ring 5. Consequently, the smaller diameter portion 14 is supported concentric with the inner ring 5. Furthermore, a plurality of apertures 17 constituting rotation side cutout portions, are formed at even spacing around the circumferential direction, so that the magnetic characteristics around the circumferential direction change alternately and at even spacing. The respective apertures 17 are of the same rectangular shape and elongate in the axial direction (the left-right direction in FIG. 1).
An opening portion at the axially inner end of the outside member 8 is covered with a cover 18 made in a bottomed cylindrical shape by deep drawing a metal plate such as stainless steel plate or aluminum plate. An annular sensor 20 is enclosed in an annular synthetic resin portion 21 inside an inner peripheral surface of a cylindrical portion 19 of the cover 18. The sensor 20 comprises a permanent magnet 22, a stator 23 formed from a ferromagnetic material such as steel plate, and a coil 24, and is formed in an overall annular shape by embedding the respective components 22, 23, 24 in the synthetic resin portion 21.
Of the respective constituent members of the sensor 20, the permanent magnet 22 is formed in an overall annular shape (ring shaped) and is magnetized radially. The inner peripheral surface of the permanent magnet 22 faces the outer peripheral surface of the portions not formed with the apertures 17 at the base end portion of the smaller diameter portion 14 of the encoder 13, across a small gap 25.
The stator 23 is formed in an overall annular shape of J-shape in cross section. An inner peripheral surface of an end portion of a radially outer cylindrical portion 26 of the stator 23 and an outer peripheral surface of the permanent magnet 22, are positioned close to or abutted against each other. An inner peripheral surface of a radially inner cylindrical portion 27 of the stator 23 opposes the portion formed with the plurality of apertures 17 at one part of the smaller diameter portion 14 of the encoder 13, across the small gap 25. Moreover, a plurality of cutouts 28 (fixed side cutout portions) are formed in the radially inner cylindrical portion 27 around the circumferential direction at a pitch (central angle pitch) equal to that of the apertures 17. Consequently, the radially inner cylindrical portion 27 is formed in a comb teeth configuration.
The coil 24 is formed in an annular shape by winding a conducting wire onto a bobbin 29 of a non magnetic material, and is located on an inner peripheral portion of the radially outer cylindrical portion 26 of the stator 23. An electromotive force induced in the coil 24 is taken out from a connector 30 which protrudes from an outer surface of the cover 18.
At the time of using the speed sensing rolling bearing unit constructed as described above, when the encoder 13 rotates together with the inner ring 5 which constitutes the inside member, the magnetic flux density in the stator 23 facing the encoder 13 changes, and the voltage induced in the coil 24 thus changes at a frequency proportional to the rotational speed of the hub 1. The reason for the change in the voltage induced in the coil 24 with the change in density of the magnetic flux flowing in the stator 23, is the same as for the case of conventional well known rotational speed sensing sensors. Moreover, the reason for the change in the density of the magnetic flux flowing in the stator 23 with rotation of the encoder 13 is as follows.
With the apertures 17 provided in the encoder 13 and the cut outs 28 provided in the stator 23, since these have the same pitch, then with rotation of the encoder 13, there is an instant where these simultaneously oppose each other around the whole periphery. Furthermore, at the instant when the respective apertures 17 and the respective cutouts 28 oppose each other, then column portions (ferromagnetic bodies) existing between the adjacent apertures 17, and tongue portions (ferromagnetic bodies) existing between the adjacent cutouts 28 oppose each other across the small gap 25. With the column portions and the tongue portions (ferromagnetic bodies) opposing each other in this way, a high density magnetic flux flows between the encoder 13 and the stator 23.
On the other hand, if the phase of the apertures 17 and of the cutouts 28 is displaced by one half, then the density of the magnetic flux flowing between the encoder 13 and the stator 23 drops. That is to say, in this condition, the apertures 17 provided in the encoder 13 oppose the tongue portions, and simultaneously, the cutouts 28 provided in the stator 23 opposes the column portions. With the column portions opposing the cut outs 28, and the tongue portions opposing the apertures 17 in this way, a relatively large air space exists around the whole periphery between the encoder 13 and the stator 23. Hence, in this condition the density of the flux flowing between the two members 13 and 23 drops. The result is that the voltage induced in the coil 24 changes in proportion to the rotational speed of the hub. By using the sensor 20 as described above, then the output voltage induced in the coil 24 changes at a frequency proportional to the rotational speed of the inside member.
With the speed sensing rolling bearing unit constructed and operated as described above, the magnetic flux output from the end face of the permanent magnet 22 of the sensor 20 always flows in the same direction inside the stator 23 of the sensor 20. Only the magnitude of the magnetic flux density changes with rotation of the encoder 13, and a voltage corresponding to this change in the magnetic flux is induced in the coil 24. Therefore it is difficult to increase the amount of change in the voltage (the difference between the maximum and minimum amount). In particular at the time of low speed traveling when the speed at which the magnetic flux density changes is low, the absolute value of the induced voltage, and the amount of change is small.
In view of this situation, there has been proposed a construction where a permanent magnet is provided on the encoder side, with south poles and north poles located on a portion of the permanent magnet facing the sensor, alternately and at even spacing around the circumference. If such an encoder incorporating permanent magnets is used, then the magnetic flux flows in the stator of the sensor alternately in opposite directions. This is referred to as alternating magnetic flux. Consequently voltages can be induced in the coil fitted to the stator, in mutually opposite directions with rotation of the encoder, thus enabling an increase in the sensor output.
Moreover, to increase the output from the sensor, it is effective to increase the diameter of the detected face of the encoder which faces the sensor. Therefore, contrary to the construction shown in the FIG. 1, the encoder is located diametrically outside of the sensor so that the diameter of the inner peripheral surface of the encoder (detected face) is increased. Furthermore, by increasing the diameter of the encoder, the number of poles provided on the encoder can be increased which is also effective in increasing the detection accuracy.
FIG. 2 shows an encoder 31 which satisfies the above requisites, and a magnetizing apparatus 33 for magnetizing a permanent magnet 32 of the encoder 31. The encoder 31 comprises an annular support ring 34 made of metal plate, and a permanent magnet 32 which is supported and secured around the whole periphery of the support ring 34. The support ring 34 comprises a smaller diameter portion 35 for attachment to a rotation ring such as the inner ring 5 (refer to FIG. 1), a larger diameter portion 36 concentric with the smaller diameter portion 35, and a ring shaped step portion 37 connecting between an and rim of the larger diameter portion 36 and an end rim of the smaller diameter portion 35. The permanent magnet 32 is formed in an overall cylindrical shape and is affixed to the inner peripheral surface of the larger diameter portion 36 around the whole periphery. Furthermore, south poles and north poles are located on the inner peripheral surface of the permanent magnet 32 alternately and at even spacing around the circumferential direction.
With the magnetizing apparatus 33 for magnetizing the permanent magnet 32 in order to make the encoder 31, a plurality of magnetizing terminals 39 are located in a cylindrical formation on an end portion (left end portion in FIG. 2) of a ferromagnetic yoke 38, at the same pitch as the pitch of the adjacent south and north poles of the permanent magnet 32, and at even spacing around the circumference. The magnetizing terminals 39 are provided in the same number as the total number of south poles and north poles located on the inner peripheral surface of the permanent magnet 32, protruding radially outward from the outer peripheral surface at the tip end of the yoke 38, with their respective outer peripheral surfaces elongate in the axial direction of the permanent magnet 32 (left to right direction in FIG. 2). Respective coils 40 are wound around the respective magnetizing terminals 39. On switching on power, the respective coils 40 radially magnetize the magnetic body (permanent magnet material, high coercive force material) which is to become the permanent magnet 32 and which faces the outer peripheral surfaces of the respective magnetizing terminals 39.
The portion surrounding the respective magnetizing terminals 39 at the tip end portion of the yoke 38 is covered with a synthetic resin portion 41, and the respective coils 40 are embedded within this synthetic resin portion 41. A location plate 42 made of a metal plate is secured to an end face of the synthetic resin portion 41. At the time of magnetizing the permanent magnet 32 to alternately locate the south poles and the north poles on the inner peripheral surface of the permanent magnet 32, the location plate 42 is abutted against the step portion 37 of the support ring 34. In this condition, the respective magnetizing terminals 39 face the inner peripheral surface of the magnetic body (permanent magnet material, high coercive force material) which constitutes the permanent magnet 32, along the full length of the magnetic body. Then in this condition, power to the respective coils 40 is switched on to magnetize the magnetic body thereby producing a permanent magnet 32 with south poles and north poles formed on the inner peripheral surface alternately and at even spacing.
When using the conventional magnetizing apparatus 33 as shown in FIG. 2 to magnetize the permanent magnet 32 of the encoder 31 having the construction shown in FIG. 2, a distance L.sub.32 between the end rim of the permanent magnet 32 and the side face of the step portion 37 of the support ring 34 cannot be made sufficiently small. That is to say, in the condition with the outer peripheral surfaces of the respective magnetizing terminals 39 of the magnetizing apparatus 33 and the inner peripheral surface of the magnetic body of the permanent magnet 32 opposing each other, the synthetic resin portion 41 in which a portion of the coils 40 is embedded and the location plate 42 exist between the end face of the yoke 38 and the step portion 37. It is therefore not possible to avoid an increase in the distance L32 by this amount.
If the distance L.sub.32 is increased, then the axial dimension of the encoder 31 is also increased by this amount, thus making it difficult to miniaturize and lighten a speed sensing rolling bearing unit incorporating the encoder 31.